The human estrogen receptor (ER) is a member of the nuclear receptor superfamily of transcription factors (Evans, Science 240:889-895 (1988)). In the absence of hormone, it resides in the nucleus of target cells in a transcriptionally inactive state. Upon binding ligand, ER undergoes a conformational change initiating a cascade of events leading ultimately to its association with specific regulatory regions within target genes (O'Malley et al, Hormone Research 47:1-26 (1991)). The ensuing effect on transcription is influenced by the cell and promoter context of the DNA-bound receptor (Tora et al, Cell 59:477-487 (1989) (Tasset et al, Cell 62:1177-1187 (1990); McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); Tzukerman et al, Mol. Endocrinol. 8:21-30 (1994)). It is in this manner that the physiological ER-agonist, estradiol, exerts its biological activity in the reproductive, skeletal and cardiovascular systems (Clark and Peck, Female Sex Steroids:Receptors and Functions (eds) Monographs on Endocrinology, Springer-Verlag, New York (1979); Chow et al, J. Clin. Invest. 89:74-78 (1992); Eaker et al, Circulation 88:1999-2009 (1993)).
In addition to these activities, estrogen has been shown to function as a mitogen in most ER-positive breast cancer cells. Thus, treatment regimens which include antiestrogens, synthetic compounds which oppose the actions of estrogen, have been effective clinically in halting or delaying the progression of the disease (Jordan and Murphy, Endocrine Reviews 11:578-610 (1990); Parker, Breast Cancer Res. Treat. 26:131-137 (1993)). The availability of these synthetic ER-modulators and subsequent dissection of their mechanism(s) of action have provided useful insights into ER action.
One of the most studied compounds in this regard is tamoxifen (Jordan and Murphy, Endocrine Reviews 11:578-610 (1990)). This compound functions as an antagonist in most ER-positive breast tumors, but displays a paradoxical agonist activity in bone and the cardiovascular system and partial agonist activity in the uterus (Kedar et al, Lancet 343:1318-1321 (1994); Love et al, New Engl. J. Med. 326:852-856 (1992); Love et al, Ann. Intern. Med. 115:860-864 (1991)). Thus, the agonist/antagonist activity of the ER-tamoxifen complex is influenced by cell context. This important observation is in apparent contradiction to long-standing models that hold that ER only exists in the cell in an active or an inactive state (Clark and Peck, Female Sex Steroids:Receptors and Functions (eds) Monographs on Endocrinology, Springer-Verlag, New York (1979)). It indicates instead that different ligands acting through the same receptor can manifest different biologies in different cells. Definition of the mechanism of this selectivity is likely to advance the understanding of processes such as tamoxifen resistance, observed in most ER-containing breast cancers, where abnormalities in ER-signaling are implicated (Tonetti and Jordan, Anti-Cancer Drugs 6:498-507 (1995)).
Using an in vitro approach, the likely mechanism for the cell selective agonist/antagonist activity of tamoxifen has been determined (Tora et al, Cell 59:477-487 (1989); Tasset et al, Cell 62:1177-1187 (1990); McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); Tzukerman et al, Mol. Endocrinol. 8:21-30 (1994)). Importantly, it has been shown that tamoxifen induces a conformational change within ER which is distinct from that induced by estradiol (McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); (Beekman et al, Molecular Endocrinology 7:1266-1274 (1993)). Furthermore, determination of the sequences within ER required for transcriptional activity indicate how these specific ligand-receptor complexes are differentially recognized by the cellular transcriptional machinery. Specifically, it has been shown that ER contains two activation domains, AF-1 (Activation Function-1) and AF-2, which permit its interaction with the transcription apparatus. The relative contribution of these AFs to overall ER efficacy differs from cell to cell (Tora et al, Cell 59:477-487 (1989); McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); Tzukerman et al, Mol. Endocrinol. 8:21-30 (1994)). Estradiol was determined to function as both an AF-1 and an AF-2 agonist, in that it exhibited maximal activity regardless of which AF was dominant in a given cellular environment. Tamoxifen, on the other hand, functions as an AF-2 antagonist, inhibiting ER activity in cells where AF-2 is required or is the dominant activator (Tora et al, Cell 59:477-487 (1989); McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); Tzukerman et al, Mol. Endocrinol. 8:21-30 (1994)). Conversely, tamoxifen functions as an agonist when AF-1 alone is required (McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); Tzukerman et al, Mol. Endocrinol. 8:21-30 (1994)). Subsequently, based on their relative AF-1/AF-2 activity, four mechanistically distinct groups of ER-modulators were defined; full agonists (i.e. estradiol), two distinct classes of partial agonists, represented by tamoxifen and raloxifene, and the pure antagonists, of which ICI182,780 is a representative member (McDonnell et al, Mol. Endocrinol. 9:659-669 (1995); Tzukerman et al, Mol. Endocrinol. 8:21-30 (1994)). These results provide a mechanistic explanation for the observed differences in the biological activities of some ER-modulators and indicate that the mechanism by which ER operates in different tissues is not identical. Interestingly, the agonist activity exhibited by ER-modulators, such as estrogen and tamoxifen, in these in vitro systems reflects their activity in the reproductive tracts of whole animals. This correlation does not extend to bone, however, where estradiol, tamoxifen and raloxifene, which display different degrees of AF-1/AF-2 agonist activity, all effectively protect against bone loss in the ovariectomized rat model. Thus, with the exception of the steroidal pure antiestrogens (ie, ICI182,780), all known classes of ER modulators appear to protect against bone loss in humans and relevant animal models, while they display different degrees of estrogenic activity in other tissues (Chow et al, J. Clin. Invest. 89:74-78 (1992); Love et al, New Engl. J. Med. 326:852-856 (1992); Draper et al, Biochemical Markers of Bone and Lipid Metabolism in Healthy Postmenopausal Women. In:C. Christiansen and B. Biis (eds) Proceedings 1993. Fourth International Symposium on Osteoporosis and Consensus Development Conference, Handelstrykkeriet, Aalborg; Wagner et al, Proc. Natl. Acad. Sci. USA 93:8739-8744 (1996); Black et al, J. Clin. Invest 93:63-69 (1994)).